Oven heater element control circuit

ABSTRACT

A switching circuit for a heating element including a power triac in series with the heater element between one line of an alternating current source and neutral. The power triac is controlled from a pilot triac connected in series with an RC network between the control electrode of the power triac and neutral. The pilot triac is triggered a short time interval after the zero crossing of the AC source and the capacitor is alternately charged in opposite directions with the charge accumulation of the capacitor being utilized to trigger the power triac.

United States Patent 1191 Gould, Jr.

[ NOV. 27, 1973 OVEN HEATER ELEMENT CONTROL CIRCUIT [75] Inventor:Robert R. Gould, Jr., Dayton, Ohio [73] Assignee: General MotorsCorporation,

Detroit, Mich.

[22] Filed: Dec. 26, 1972 21 Appl. No.: 318,012

52 us. c1. 219/501, 307/252 [51] Int. Cl. H05b 1/02 [58] Field of Search219/494, 501, 497,

219/505; 323/1 SW, 2 S, 24; 307/252 [56] References Cited UNITED STATESPATENTS 3,484,623 12 1969 Cain 323/24 UX 3,646,577 2 1972 Ernst 219 5013,512,077 5 1970 EvaldsQ. 219 494 x CONTROL :CIRCUIT 12/1970 Welch219/501 Fricker et al. 323/24 X Primary Examiner-Bernard A. GilheanyAssistant Examiner--F. E. Bell Attorney-Eugene W. Christen et al.

[5 7] ABSTRACT A switching circuit for a heating element including apower triac in series with the heater element between one line of analternating current source and neutral. The power triac is controlledfrom a pilot triac connected in series with an RC network between thecontrol electrode of the power triac and neutral. The pilot triac istriggered a short time interval after the zero crossing of the AC sourceand the capacitor is alternately charged in opposite directions with thecharge accumulation of the capacitor being utilized to trigger the powertriac.

2 Claims, 3 Drawing Figures t1 OVEN?BEA-P ER?EIZEEMEIIT CONTROL CIRCUIT.This inventiona'relates to .synchronous switching cir-"Semiconductoriswitches such as the triac. have been fproposed'inrecent-years tocontrol the supply of alternating,currentpower tdtheresistive. heating elements of a domestic. rangeJT he triac: normallyexhibits a high impedance 1 characteristic between two main currentcarrying terminals but maybe-switched to a low imped- 'ancestate by theapplication of a triggering impulse to its .gateaterminal.:Bygtriggering the triacrat the zero crossover point of i the AC powersupply, continuous AC power may 'be applied to'the load-with minimum:radio frequency interference. Accordingly,'the trigger circuits'forthetriacusually include a 'zero crossover switch cooperating with atemperature sensing circuit for firing the triac at thezero crossoverpoint whenever the oven temperature is below a preselected temperature;Control circuits'of this nature are disclosed in the US. Pat. Nos.3;546,435'to Welch, and 3,665,159 to Decker et a1. Where the controlcircuit is referenced to therneutralline of the domestic supply and theload is -connected line-'to-line, transformers are necessary to provideeffective'electrical isolation of the control circuitfromthehighvoltagepower supply-or current limiting resistors must-be connectedin' thegatecircuit of the triac in ordertolimitithe current flow throughthetriac; gate. Transformersare bulky and expensive and 'the use ofcurrentlimiting resistors results in the dissipation ofconsiderableamounts of power.

With the foregoing inrnind it is an object of the present invention toprovide an improved control circuit for triggering abidirectional=semiconductorswitc'h'providing transformerless isolationand reducedpower 'dissipation.

In accordance with the present invention a power triac connectedinseries with the oven heating element Referring now to the drawings andinitially to FIG. 1, the numerals:land.-12 designate electricalresistance heating elements such as the Broil and Bake heating elementsrespectively of an oven.

Current is supplied to the elements and 12 from the standard three wiredomestic power supply nominally of.2 40 volt 60 cycle altematingcurrent.The dcmestic supply has a pair .of line wires L1 and L2 and a groundedneutral conductor N. The Broil element 10 is connected between L1 and L2through a thermal relay generallydesignated 14 having a heater element16 and a bimetal switch 18, 20 and a bidirectional curline'L2. The gateterminals 26 and 28 of the triacs 22 and-24 are connected with theneutral line N through firing circuits generally designated 30 and 32respectively.Each of the firing circuits 30 and 32 include a 0.022microfarad'capacitor 34, 36, and a 1,200 ohm one-half watt resistor 38,40 and a pilot triac 42, 44. The heater element of the relay 14 isconnected between line' L1 and neutral through a trio 46. The triacs 42,44, and 46 are the L4000K7 1.6 amp, 400 volt rated devices availablefrom Electronic Control Corporation.

The control circuit for firing the pilot triacs 42, 44, and 46 isgenerally designated 48. The control circuit 48 is connected between theline L2 and the neutral conductor N and includes a zero crossoverdetector for providing an indication of when the AC supply reachesacross the power supply has its gate terminal connected to the'neutralline of thepower supply through a series network including a resistor, acapacitor, and a pilot triac. Thegate'of the:pilot triac is connectedwith control circuitry producing a seriesof triggerpulses synchronizedwiththe zero crossover voltagepointof the line-to-neutral voltagewhenever the temperature of theoven is below the; preselectedtemperature. The capacitor is alternately charged in opposite directionsas the pilot triac is successively triggered. The charge accumulation onthe capacitoris utilized to trigger the power triac at thesubsequentzero crossover voltage pointito provide full wave current-totheload.

Otherobjects-and advantagesof the presentinvention may be had fromthefollowing detailed description which should be taken in conjunction withthe drawings in which:

FIG. 1 is a schematic diagram partially in block diagram showing thepowertriacfiring circuit of the present invention;

for controlling the gating'of the pilot triac in the power triac firingcircuit. of FIG. 1.

the zero crossover voltage point. The circuitry 48 further includescircuit means responsive to the zero crossover detector forproviding a30 microsecond 44, and 46. The control circuitry 48 further includes Itemperature sensing means which cooperate. with the zero crossoverdetector so that the triggering pulses are applied to the gates of thetriacs 42, 44 only when the temperature in the oven is below apreselected temperature. During the Broil operation the triac 42 istriggered at each zero crossing of the AC supply. During the Bakeoperation the triac 44 is triggered each zero crossing of the AC supplyand in addition the triac 42 is triggered for two out of each eight zerocrossings to provide 25 percent operation of the Broil element 10 on anintegral cycle band.

The operation of the trigger circuits 30 and 32 are identical and willbe described with reference to the trigger circuit 30 and the waveformsshown in FIG. 2, assuming that the capacitor 34 is initially in adischarged state. The pilot triac 42 is triggered approximatelymicroseconds after the zero crossover point -of the AC supply when theline voltage is approximately 7 volts. The short delay is provided sothat there is enough power supply voltage available to provide theholding current necessary for the pilot triac 42. When the triac 42 istired the capacitor 34 is charged through the resistor 38. As the linevoltage reaches its peak Y value the charging current 1 drops to zeroand the pilot triac 42 turns off. As the line voltage drops toward thenext zero crossover point the pilot triac 42 being in a nonconductivestate holds the charge on the capacitor 34. 100 microseconds after theAC power supply reaches the zero crossover point the pilot triac 42 isagain fired and causes the capacitor 34 to discharge to provide a firingcurrent through the gate 26 of the power triac 22. In the I waveform thepeak position is extended on the time coordinate for illustrativepurposes. The peak current of approximately 140 milliamps actuallyoccurs shortly after the pilot triac istriggered as determined by thestray inductance of the circuit. As the line voltage proceeds on withits negative excursion the capacitor 34 is completely discharged andproceeds to charge in the opposite direction. As the power supplyvoltage reaches the peak negative excursion the charging current in thecapacitor drops off to zero and the pilot triac 42 again turns off andholds the charge on the capacitor 34. As the power supply voltage againpasses the zero crossover point from the negative direction the storedcharge in the capacitor 34 is released upon firing of the pilot triac 42to trigger to the power triac 22 thereby providing full wave current tothe Broil element 10.

During the Bake mode of operation the Bake heating element 12 isenergized in the same fashion as previously described with regards tothe Broil element 10. In addition the Broil element is energized 25percent of the time by providing triggering of the pilot triac 42 on twoout of eight zero crossings of the AC'power supply as will be explainedin detail in connection with the description of the circuit of FIG. 3.

Referring now to FIG. 3 the control circuitry 48 includes a zerocrossover detector 50 connected between the line L2 and the groundedneutral wire N of the power. supply. The detector 50 is a conventionaldevice such as the Fairchild A 742 and provides a series of pulsessynchronized with the zero crossover voltage point of the AC supply.connected thereto. The output of the detector 50 is delayed, forexample; by 100 microseconds by a delay network 51 such as aretriggerable one-shot and provides one input to an AND gate 52 whichcontrols a retriggerable monostable multivibrator or one-shot 54. Theone-shot 54 determines the width of the pulse applied to the gate of thepilot triacs 42 and 44. A minimum pulse width of 30 microseconds isprovided by the one-shot 54. The other input to the AND gate 52 is froma temperature setting and control circuit generally designated 56. Thecircuit 56 comprises a temperature sensing A/D converter 57 including areference oscillator 58 and a sense oscillator 60. The referenceoscillator 58 is a conventional oscillator the output frequency of whichis determined by its internal RC network. The sense oscillator 60 is aconventional oscillator having an internal RC network where thecapacitor is of the same value as that of the reference oscillator butthe resistor is temperature sensitive and has a positive temperaturecoefficient of resistance so that as the oven temperature increases thefrequency of the sense oscillator 60 decreases. The pulsating output ofthe oscillators 58 and 60 are applied to respective counters 62 and 64.The counters 62 and 64 are controlled by a sample clock 66 producing asquare I wave output of substantially lower frequency than the outputfrequencies of either the reference oscillator 58 or the senseoscillator 60. When the output of the clock 66 is low the counters 62and 64 are preset so that their outputs are low. When the clock 66 goeshigh the counters 62 and 64 accumulate the output pulses of theoscillators 58 and 60 respectively and upon reaching a terminal countare disabled so that no further counting occurs until the clock 66returns to its high state after presetting the counters 62 and 64. Theoutput of the counter 62 is applied to the reset terminal of decadecounters 68 and 70 and 72. The counters 68, 70, and 72 provide at theiroutput the BCD equivalent of the units, tens, and hundreds digitsrespectively of the actual temperature of the oven as represented by thedifference in frequency between the two oscillators 58 and 60. Thecounter 68 is clocked from the reference oscillator through a NOR gate74 and inverter 76. The NOR-gate 74 is controlled by the outputof thecounter 64 which remains low until the counter 64 reaches its terminalcount. The decade counter 70 is clocked from the counter 68 on the tenthclock pulse to the decade counter 68, and similarly, the decade counter72 is clocked from the decade counter 70 on the tenth clock pulse to thedecade counter 70. The counters 68, 70, and 72 are released to commencecounting when the counter 62 reaches its terminal count and continuecounting until the counter 64 reaches its terminal count at which timethe gate 74 is disabled. The analog-todigital converter 57 thus providesa digital indication of the analog temperature of the oven.

Temperature input elements 78, 80, and 82 which may be conventional BCDthumb wheel switches, are provided for setting respectively the units,tens, and hundreds digits of the desired oven operating temperature. TheBCD outputs of the switches 78-82 and the decade counters 68-72 arecompared in a comparator 84 which provide a control output whichswitches from a logic l to a logic 0" whenever the actual temperature ofthe oven as represented by the counters 6872 is less than the desiredoven temperature as represented by the output of the switches 78-82. Theoutput of the comparator 84 is connected with the D input of a flip-flop86, the 6 output of which provides the second input to the AND gate 52.Whenever the temperature of the oven is less than the desiredtemperature, the 6 output of the flip-flop 86 will be high and themonostablemultivibrator 54 will be triggered through the microseconddelay network 51 by each of the output pulses of the detector 50. Theoutput of the multivibrator 54 enables a second monostable multivibrator88. The multivibrator 88 is enabled for only two of eight output pulsesof the detector 50 under the control of a ripple counter comprisingflip-flops 90, 92, and 94 and an AND gate 96. The inputs to the gate 96are from the multivibrator 54 and the Q outputs of the flip-flops 92 and94. The multivibrator 88 is triggered from the multivibrator 54 but isdisabled by the logic 0 input to the gate 96 from the flip-flops 92and/or 94, after the first two of each eight output pulses of themultivibrator 54.

The selection of the Bake or Broil operation may be accomplished by aselector switch having contacts designated BK and BR. During the Bakeoperation the contacts BK of the switch are closed so that the output ofthe multivibrator 54 is applied to the gate terminal of the pilot triac44 through a resistor 98 and the output of the multivibrator 88 isapplied to the gate terminal of the pilot triac 42 through a resistor100. Accordingly, continuous power is supplied to the Bake element 12and 25 percent of the power is applied to the Broil element 10. Duringthe Broil operation the contacts BR of the switch are closed and thecontact BK are opened so that the Broil element is energized at each 0crossing of the'AC supply as long as the temperature of the oven isbelow the set temperature. The triac 46 is triggered from amultivibrator 102 during either the Bake or Broil operation to applypower to'the heater 16 of the thermal relay 14 to close the contacts 18,20 when either the Bake or Broil operation is initiated.

Having thus described my invention what I claim is:

1. A circuit for controlling the current flow through a load from an AC.voltage source having first and second line terminals and a groundedneutral terminal comprising;

first and second gate controlled bilateral semiconductor switch meanshaving first and second load terminals and a control gate terminal;

means connecting the load terminals of said first switch means in serieswith said load between said first and second line terminals of saidsource;

a series resistance capacitance network connecting the gate terminal ofsaid first switch means to said first load terminal of said secondswitch means, means connecting said second load terminal of said secondswitch means to said neutral terminal of means applying the output ofthe control circuit means to the gate terminal of said second switchmeans.

2. A circuit for controlling the supply of an alternating currentvoltage source to an electric oven, said circuit comprising:

an oven heating element, a first triac connected in series with saidheating element and having a reference terminal connected to a firstline terminal of said source and its other load terminal connected withsaid heating element, means connecting the other side of said heatingelement to the second line terminal of said source;

a second triac and an RC circuit connected in series between the gateterminal of said first triac and the neutral terminal of said source;

control circuit means including a zero crossover detector connectedbetween one-line terminal of said source and the neutral terminal ofsaid source for providing a series of pulses synchronized with a zerocrossover voltage point of said source, means for setting a desiredtemperature in said oven, means for sensing the temperature of saidoven, and means responsive to the output of said zero crossover detectorfor triggering said second triac a predetermined time intervalsubsequent to the zero crossover point of said source whenever thetemperature of said oven is below the set temperature.

1. A circuit for controlling the current flow through a load from anA.C. voltage source having first and second line terminals and agrounded neutral terminal comprising; first and second gate controlledbilateral semi-conductor switch means having first and second loadterminals and a control gate terminal; means connecting the loadterminals of said first switch means in series with said load betweensaid first and second line terminals of said source; a series resistancecapacitance network connecting the gate terminal of said first switchmeans to said first load terminal of said second switch means, meansconnecting said second load terminal of said second switch means to saidneutral terminal of said source; neutral referenced control circuitmeans for developing a series of output trigger pulses synchronized withthe zero crossover voltage point of said source; and means applying theoutput of the control circuit means to the gate terminal of said secondswitch means.
 2. A circuit for controlling the supply of an alternatingcurrent voltage source to an electric oven, said circuit comprising: anoven heating element, a first triac connected in series with saidheating element and having a reference terminal connected to a firstline terminal of said source and its other load terminal connected withsaid heating element, means connecting the other side of said heatingelement to the second line terminal of said source; a second triac andan RC circuit connected in series between the gate terminal of saidfirst triac and the neutral terminal of said source; control circuitmeans including a zero crossover detector connected between one-lineterminal of said source and the neutral terminal of said source forproviding a series of pulses synchronized with a zero crossover voltagepoint of said source, means for setting a desired temperature in saidoven, means for sensing the temperature of said oven, and meansresponsive to the output of said zero crossover detector for triggeringsaid second triac a predetermined time interval subsequent to the zerocrossover point of said source whenever the temperature of said oven isbelow the set temperature.